­Chances are, your cell phone is tethered to one particular service provider. If you try to leave that company, your phone won't work. It's locked. If you travel to another country and try to use another company's cell phone service on your old phone, that won't work either. Locked again.

It doesn't have to be this way, though. Depending on what kind of phone you have and what company you purchase cell phone service from, you might be able to unlock your phone. Then, you'll have the freedom to use the same phone with different companies, or switch phone numbers and service accounts.

­If you've ever wondered what the difference between a locked and an unlocked cell phone is, or if you've ever wondered if your phone can be unlocked, or how you can do it, we'll explain it to you in this article. Plus, we'll tell you why having an unlocked phone can be a good thing, and whether or not it's legal.

Cell Phone Unlocking: Can your phone be unlocked?

SIM cards are small chips about the size of your thumb, but it's the key to unlocking a locked cell phone.

There are two cell phone technologies that are used by the majority of the world's mobile phone service providers: Global System for Mobile communication (GSM) and Code Division Multiple Access (CDMA). If you have a CDMA phone, then your phone is not unlockable. End of story. However, if your phone operates on a GSM network, then it might be unlockable.

What's the difference? GSM phones use subscriber identity module (SIM) cards. A SIM card is a small card that's inserted into the phone. It contains all your contacts and settings, and it's linked to your account. You can take the SIM card out, put it into another phone, and if someone calls your number, the new phone will ring. You can also put a different SIM card in your unlocked phone, and your phone will then work with whatever phone number and account is linked to that card. CDMA phones have no SIM cards and must be authenticated by the service provider, which makes unlocking a phone impossible [source: Segan].

If a phone is locked, the service provider has installed some software on the phone that ties the subscriber ID number on the SIM card to the serial number of that particular phone. If the SIM card and phone serial number don't match, the phone simply won't work. The SIM card won't work in other phones, and the phone won't work with other SIM cards.

If you live in North America, you can usually determine whether your phone is GSM or CDMA based on the service provider you use. T-Mobile and AT&T use GSM, while Verizon and Sprint use CDMA. In Europe, almost all phones are GSM.

How can you tell if your phone is already unlocked? In Europe, unlocked phones are more common. In fact, many countries have cell phone portability laws that make it illegal for a company to lock a phone to a specific account (though these laws seem to change frequently). In North America, fewer phones are sold unlocked. If you received a phone for free or at a very low price when you signed a contract for your cell phone service, your phone is almost certainly locked.

There's an easy way to find out for sure: Find a friend whose phone is known to be unlocked. Have your friend place his or her SIM card in your phone, and then call your friend's number. If your phone rings, you know your phone is unlocked. If your phone simply generates an error message, then it's locked.

Next, we'll explain the advantages of having an unlocked phone, and explain how to make it happen.

How to Unlock GSM Phones

Replacing the SIM card in a cell phone is easy, but it only works if the phone is unlocked.

There are two reasons to unlock a phone. First, it gives you the freedom to switch between service providers. Assuming a long-term service contract doesn't stand in your way (along with the contract termination fees that come along with it), you can change service providers as often as you like and never have to buy a new phone. Just put in a new SIM card supplied by your new service provider and your old phone will work just fine. You will have to go through the effort of migrating your old phone number to the new account, however.

The other reason involves traveling to Europe or Asia. If you use your North American phone while traveling, you will rack up enormous roaming fees. International cell phone companies don't all rely on the two-year contract business model favored in the United States. You can buy a SIM card for your phone that's pre-loaded with a fixed number of minutes that will work in whatever country you'll be spending time in. Some European companies even offer SIM cards whose accounts work across much of the continent, not just in one country. Frequent travelers or even those taking occasional vacations will enjoy significant savings with an unlocked phone.

The process of unlocking a phone is actually quite simple, and doesn't require any technical knowledge. For some phones, you just have to enter a numeric code into the phone. For a fee, some service providers will even give you the proper unlock code for your phone. There are also third party companies that provide cell phone unlock codes for a fee that ranges from $5 to about $25, depending on the model of phone you have.

Some phones need to be connected to a special device through the phone's data port to be unlocked. Again, a few service providers offer this service, but most phone owners will have to turn to third parties to accomplish this. You pay them a fee, send them your phone, they unlock it and send it back.

But what if you want to unlock your iPhone? We'll explain that in the next section.

How to Unlock Smartphones

Smartphones present a slightly different unlocking scenario. They're essentially handheld mobile computers. Locking and unlocking them is a more involved process than the simple linking of serial numbers and account numbers seen in regular cell phones.

Smartphones are often locked to a service provider. For example, the iPhone is locked to the AT&T network. For some smartphone owners, unlocking isn't a matter of wanting to change service providers. These owners just want to gain control over the applications they can install on their device. A lot of smartphone manufacturers lock the devices so that only approved applications can be installed. The iPhone can only install apps purchased from Apple's App Store, for instance. If there's an application you want to install that isn't officially approved and offered by Apple, you're out of luck. That is, unless you can unlock your smartphone.

It's possible to purchase unlocked iPhones, or you can apply a software crack to an iPhone to unlock it. This is often referred to as jailbreaking the phone. Because the software on a smartphone is more complicated than a cell phone's, the unlocking process is more difficult than simply entering a code. The software cracks can have unpredictable results, rendering some features of the phone (or the phone itself) non-functional.

Is all this unlocking and jailbreaking legal? In the next section, we'll find out.

Acronyms Explained

GSM stands for Global System for Mobile communications. It was developed in Europe, and the original name is French: Groupe Spécial Mobile. CDMA stands for Code Division Multiple Access. It is actually a radio technology that has been adapted for cell phone use. You may also have heard references to TDMA, Time Division Multiple Access. This is a different way of dividing cell phone signals over a given amount of bandwidth -- GSM is actually a "flavor" of TDMA.

Is Cell Phone Unlocking Legal?

Traveling in Europe without getting a SIM card from a local phone company can lead to expensive roaming charges.

­The last question everyone asks about cell phone unlocking: is it legal? In Europe, the answer is generally yes. Though laws differ from country to country, they tend to favor consumers over companies. In the United States, the answer is also yes. The U.S. Copyright Office issues rulings every three years, and in 2006, they declared that unlocking a cell phone does not infringe on the copyright of the phone manufacturer or service provider, and therefore isn't prohibited [source: Ars Technica].

In the ruling, they suggested that locking phones to accounts only serves to support a particular business model. "The underlying activity sought to be performed by the owner of the handset is to allow the handset to do what it was manufactured to do -- lawfully connect to any carrier. This is a noninfringing activity by the user...the purpose of the software lock appears to be limited to restricting the owner's use of the mobile handset to support a business model, rather than to protect access to a copyrighted work itself." [source: U.S. Copyright Office].

That ruling could be reversed later in 2009, but the justification for the original ruling hasn't changed, so it may stand. However, in 2008, Apple filed an opposition to the ruling, asking that it be overturned in 2009. They want jailbreaking iPhones to be illegal [source: U.S. Copyright Office].

There is one caveat, however. Unlocking a phone might violate the terms of any contract you may have signed with your service provider. If so, you could be subject to whatever penalties are outlined in the contract, or your service may be cut off. It's also unclear if it's legal to offer third-party cell phone unlocking services -- U.S. courts have not yet ruled on this matter.

For more information on unlocking cell phones, check out the links on the next page.

More Great Links

Sources

Apple, Inc. "In the matter of Exemption to Prohibition on Circumvention of Copyright Protection Systems for Access Control Technologies." 2008. http://www.copyright.gov/1201/2008/responses/apple-inc-31.pdf